If Velupillai Prabhakaran, the commander of the Tamil Tigers and the most feared guerrilla leader in Southern Asia, wanted to take the Sri Lankan army totally off-guard, he could do worse than recruit a few foreigners to set off his bombs. That was the idle thought going through my mind as I was waved through one army road-block after another on the route north from Colombo to Vavuniya, the scruffy little garrison town that is the front line in Sri Lanka's endless war.

North of Vavuniya is the territory stubbornly held by Mr Prabhakaran's fanatical army, the Liberation Tigers of Tamil Eelam (LTTE). And it was only at the northern edge of the town, where government-held land blurs off into no man's land strewn with "Johnny mines", the anti-personnel devices invented by Mr Prabhakaran himself, that I was obliged to turn around. "Very dangerous!" a gesticulating soldier shouted at me. "LTTE everywhere!"

In December 1995 the principal city of the north, Jaffna, was captured from the Tigers by the Sri Lankan army, along with much of the surrounding countryside. Up to half a million of Jaffna's residents, made into refugees by the war, have since returned home. But Jaffna's link with the south is still in the hands of the Tigers, whose base is deep in the jungle of the north-east. Today the only ways of getting to Jaffna are by the government's frighteningly ill-maintained military aircraft, or by ships which are frequently attacked by the Sea Tigers, Mr Prabhakaran's marine arm. So the rehabilitation of Jaffna has yet to get properly under way. Accordingly, in May President Chandrika Kumaratunga ordered an offensive to capture the road all the way north.

But already the offensive has become bogged down, just a few kilometres north of Vavuniya. Two ferocious Tiger counter-attacks in June stopped the government forces in their tracks. In Colombo now the folly of trying to defeat a guerrilla force by conventional means, and of trying to hold a narrow strip of road while guerrillas infest the countryside on both sides, is the small change of conversation.

Now that "Black July" has arrived, the government has been even more wary of exposing its troops to attack. July is studded with evil anniversaries here. While the beginning of inter-communal violence can be traced back to the early 70s, the critical escalation occurred on 23 July 1983, when the Sinhalese population erupted in violent attacks against Tamils, leaving hundreds dead and injured. Four years later, on 5 July 1987, an LTTE guerrilla drove a truck full of explosives into an army billet and blew it up, inaugurating the suicide tactic which culminated in 1991 in the assassination of Rajiv Gandhi, the former Indian prime minister. It was in July, too - on 17 July 1996 - that the LTTE demonstrated their undiminished power, killing more than 1,300 government troops in the coastal town of Mullaitivu, in the the north-east.

So far this month two MPs have been murdered. Yesterday police blamed the LTTE for the killing of an opposition politician and five other people, including a four-year-old boy, near the eastern town of Trincomalee. Mohammed Moharoof of the United National Party was visiting villages where the rebels had kidnapped fishermen in recent weeks. Another MP, Arunasalam Thangathurai of the Tamil United Liberation Front was killed in a grenade attack on 5 July.

Few can foresee an end to the war which consumes more than 20 per cent of the budget and has badly damaged the tourist industry, even though none of the most important resort areas has been affected in recent years.

With little serious threat to his jungle stronghold, and with a military machine that remains impressive, it is hard to see what could persuade Velupillai Prabhakaran to return to the negotiating table.